Method for determining focus and quality of searchlight reflectors



March 1957 J. A. LE RECHE ET AL 2,784,640

METHOD FOR DETERMINING FOCUS AND QUALITY OF SEARCHLIGHT REFLECTORS Filed March 15, 1954 as INVENTORS JOHN A. LE RE CHE THEODORE H. EG/E0701? F I G. 3 V '1 BY jug/W ATTORN YS United States Patent METHOD FOR DETERMINING FOCUS QUALITY OF-SEARCHLIGHT REFLECTORS john A. Le Reche, McLean, Va andTlleodore H. Projector, Forest 'Glen, Md assignor's to w the United States flme i a r p e ented tl e ecre r f e y Application March 15, 1954, SeriaLNo. 416,448 6 Claims. (Cl. Sh -+14 (Granted under Title 35, U. S. Code (1952), see. 266) The invention deseribedherei may be manufactured a d used by or for the, Government of; the United. States of America for governmental purposeswithout the paymcnt of any royaltiesthereon ortherefor.

This invention relates to arnethod, for simultaneously determining the 0 311 point andmvaluating the, quality of Searchlight reflectors, at short range;

High intensity searchlights, intended for use at great distances, usually consist of a parabolic reflector and a small light source of very high luminance placed at the focal point of the reflector. Ifthe light source positioned at the reflector focal point were infinitely small and the reflector perfect, then the emergent beam would consist of parallel rays of light and the cross section of the beamat anypoi nt would be constant in shape and size. Actual light sources, however, are alwaysvfinite insize and reflectors depart more or less from perfect parabolic form. Becauseof these considerations an element of uncertainty enters into the accurate determination of the point, in any givenreflector at which the light source must be placed to obtain maximum performance.

In most Searchlight applications, particularly when a single concentrated light source such as a carbon arc is utilized, maximum performan ce is defined as the performance obtained when the axial candlepower is at a maximum. The most direct and unambiguous determination of the focal adjustment of the light source. can therefore. best be made by photometric measurement. of the candle power of the beam. In this method, the light source is moved along the axis of the optical system until the position oflmaximum beam candlepower has been found, and this position is recorded in terms, of any easily measured distance between convenient points on the light source and the reflector. Photometric measure ments of this type must be made at considerable distarices, sometimes of the order of several hundred feet, and may involve other requirements that render the method unsuitable for. many field installations.

Another method of determining the focal point of a parabolic reflector is to utilize a small brightly lit target, such asa lamp, which is effectively positioned at infinity with respectto the reflector. A. small card is placed within. the reflector and is moved back and forth until the clearest image of the distant object is obtained on the card. The point at which the clearest image is ob tained is the focal point of the reflector. This position of the card is recorded in terms of any easily measured distance between convenient points on the card and the reflector.

Both of thevaforementioned methods of determining the focal point of a reflector have the disadvantage that a large testing distance is required. The instant invention, however, sets forth a method for simultaneously testing both the focus and quality of searchlight reflectors at short range without the utilization of specialized complex equipment. The method disclosed by the instant invention, therefore, is ideal for use in applications where 2,784,640 a e ed M n 12 1 -57 ice pa l t a em um a d her mp e testing. quiv ment is, not. available, i

The, focal point determination aspect of the. instant invention is based on theprinciple that if a smalllight source which provides as nearly as pos sible a.point source of light is positionederactly at the focal point of a true parabolic reflector all of the l'ight rays which are reflected by the reflector willbe substantially parallel. In this inentio e e, an aq 11 itfiat u Pla a k is positionedin parallelism with the aperture plane of the reflector. A planar screen, having an e i t outline of said mask formed ithereon, is positioned a few feet from the aperture plane. of the reflector and. in parallelism with said aperture plane and with said mask. The. opaque mask and the outline of, the plane are positioned in such aimanner that when the light source is moved to the focal poin t of the refle ctor, so that parallel rays of light areemitted from the reflector, theshadow formed by the mask will fall within the confines of the traced outline of the maslglon the, screen, It. can be readily seen that when the light source is not at the focal point of the reflector but lies between, the focal point of the reflector and the mas]; that the shadow. of themask formed onthescreenwill be smallerthan the. outline of the mask. On the other hand, when the light source lies between, the focal point and the wallofthereflector, along the, axis of the reflector, the shad-ow of the mask formedon the screen will be larger than the outline of the mask on thescreen. Only when the shadow formed by the mask and the outline of the mask substantially coincide on the screen is the light source at thefocal point of the parabolic. reflector, At this point. offlcong'ruehee. the position of the light source can be measured, and is the focal point of a particular reflector.

The reflcctor quality determination aspect 'of this invention, which is simultaneously carried out with the focalpoint determination, consists of observing the extent ofdeparture of the above mentioned. mask shadow and outline onthescreen fronr exact congrue ce when the best coincidence, is obtained, This departure is one measure of the quality of the reflector. Another measure of reflector quality is" the uniformity of the light thrown on the screen. For example lackof suflicient reflector polishing would be. indicated. by the appearance of shadows or light patches on the screen corresponding geometrically to the imperfections in the reflector.

It is therefore one object of this invention to, provide a quick convenientmethod for determining the focal point of Searchlight reflectors which, requires little space, simple equipment, and which. can be, performed by unskilled personnel.

It is another object of this invention to disclose a method and apparatus. for simultaneously evaluating the optical quality of searchlight reflectors, while the focal point of the reflector is being determined, without the use of complex precision optical equipment.

Other objects and many of the attendant advantages of this invention will be readily appreciated as the, same becomes better understood by reference to the following detailed description when considered in, connection with the accompanying drawings wherein:

Fig. l is a schematic diagram disclosing a portion of the apparatus set up for testing a parabolic reflector;

Fig. 2 is a diagram which discloses another portion of the apparatus which is used during the testing of a parabolic reflector;

Fig. 3 is a diagram which aids in explaining how the test results may be evaluated; and

Fig. 4 is an enlarged detail view which shows a bracketsuitable for attaching a mask to a reflector.

Reference is now made to Fig. 1 wherein numeral 10. designates a basesupon. whichnparabolic..reflectorull is adapted to be'mounted by means of supports 12. Pedestal 13 is also mounted on base 10. Journalled for rotation within pedestal 13 is shaft 14 having pinion 1S aflixed thereto. Mounted for sliding movement through pedestal 13 is rod 16 which has a portion thereof formed into rack 17. Pinion is adapted to mesh with'rack 17 to thereby cause rod 16 to move in the direction of the arrows, Fig. 1, when shaft 14 is rotated.

Rod 16 fits through an opening (not shown) in the rear of reflector 11. Mounted on the portion of rod 16 within reflector 11 is an extremely small ophthalmic lamp 18 which is suitably connected to a source of electric current (not shown). The purpose of using an extremely small lamp is to approximate a light source which is as close to the theoretical point source of light as possible. Ophthalmic lamp 18 lies on the optical axis of parabolic reflector 11 and is adapted to move back and forth along this axis when shaft 14 is rotated.

It is to be noted at this point that the apparatus for moving lamp 18 back and forth and for holding the reflector 11 has been shown schematically for the purpose of explaining this invention. It is to be therefore understood that any suitable means for moving lamp 18 back and forth and for supporting reflector 11 can be used so long as they accomplish the required result. It is also to be noted that the lamp supporting structure can be inserted into the reflector through the aperture plane of the reflector when the construction of the reflector requires this procedure.

Mask 19, which is constructed of any suitable opaque material, has brackets 20 suitably attached thereto. These brackets are adapted to fit over the lip of reflector 11,Figs. 1 and 4, for the purpose of holding mask 19 in parallelism with the aperture plane 27 of parabolic reflector 11.

Attention is now directed to Fig. 2. Reflector 11 is shown in dotted lines. Attached to reflector 11 in the manner described above is opaque mask 19. Screen 21 is positioned a few feet from and parallel to both mask 21 and to the aperture plane of reflector 11. Screen 21 has formed thereon an exact outline 22 of mask 19. In the operation of the apparatus lamp 18 is moved back and forth along the optical axis of reflector 11, Fig. 1. If the reflector 11 were perfectly formed and light source 18 were a perfect point, then the shadow 23 which would be formed by the mask on the screen, when the lamp 18 was at the focal point of the reflector, would fall Within lines 22 on the screen, as shown in Fig. 2. The theory for this result is explained above. However, in actual practice, shadow 23 will not be exactly congruent with outline 22. Therefore, as a practical matter, when the best coincidence of shadow 23 and outline 22 is obtained, lamp 18 will, for all practical purposes, be at the focal point of reflector 11. This position can be measured and gives the focal point of the particular reflector under test.

Attention is now directed to Fig. 3 for a discussion of how the quality of the reflector may be evaluated. It can be seen from this figure that under conditions of best coincidence the shadow 24 of mask 19 may not fit exactly within the confines of outline 22. This is an indication that the reflector is not truly parabolic. It is to be further noted that certain edges of the mask shadow such as 25 may not be perfectly straight as are the edges of mask 19. This is also an indication that there is a departure of the reflector from true parabolic form. It is to be noted in this respect that mask 19 is in the shape of a square and that the condition of the edges of the shadow of this square which are formed on the screen give an indication only on the condition of those portions of the reflector which reflect the light which forms these edge portions of the shadow. Thus, in the foregoing sense, only a relatively small fraction of the total area of the reflector is actually utilized in the test. This portion consists of the thin strips on the reflector I short range.

that contribute to the formation of the edges of the shadow formed by the mask. It is apparent that in using a square-shaped mask, sampling of the reflector areas may be accomplished at a variety of radii from the axis of the reflector bounded by the shorter distance across the square opening in the mask to the wider distance across the outermost corners thereof, such area including, if desired, the most effective area of the reflector. However the mask 19 can be designed in any way to provide a sampling of areas of the reflector which are particularly critical. Furthermore, the mask may be designed in a variety of shapes which will provide a sampling of areas of the reflector having difierent radii. It can also be seen from Fig. 3 that irregular concentric lines such as 26 may be formed on the screen within the light beam which is projected on the screen. The appearance of lines 26 is an indication that the reflector has not been sufficiently polished after the grinding op eration and are the result of corresponding hills and valleys in the reflector surface caused by the grinding operation and not removed by polishing. It can thus be seen that the foregoing tests results give an indication of the condition and quality of a parabolic reflector at It can further be seen that these results are obtainable with a minimum of equipment.

It is to be noted from the foregoing explanation that the projected shadow pattern reveals reflector imperfections clearly. In addition to revealing the distortion by deviations between the edge of the shadow of the mask and its traced outline on the screen, an examination of the edge of the beam and of the appearance of the unshadowed areas within the beam will clearly show excessive prominent grinding marks, bumps or hollows, distortions due to thermal or mechanical stresses, etc., such as those indicated by lines 26 of Fig. 3. While reflectors may be examined visually for such distortions to a limited extent, the present method makes the detection of these distortions much more thorough. Moreover with the projected shadow pattern, quantitative analysis of the defects may be made quickly and accurately.

I Obviously many modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.

Having thus described our invention we claim:

1. A method of determining the focal point of a reflector which will emit parallel rays of light when a small light source is positioned at the focal point thereof comprising the steps of: substituting a small point light source for the light source normally to be used with said reflector, positioning a mask having at least one opening therethrough in front of said reflector to intercept the light rays reflected therefrom, positioning a screen in front of said reflector to traverse the same reflected light rays as said mask, said screen being positioned on the opposite side of said mask as said reflector and in spaced and parallel optical relationship thereto, placing an exact outline of said mask on said screen, and moving the light source along the optical axis of said reflector until a shadow of said mask is congruent with said outline on said screen, said light source then being at the focal point of said reflector.

2. The method set forth in claim 1 and further comprising the step of: measuring the distance from the light source to the center of the reflector when the shadow of the mask is congruent with the outline on the screen.

3. A method of simultaneously determining the focal point and quality of a reflector which will emit parallel rays of light when a small light source is positioned at the focal point thereof comprising the steps of: substituting a small point light source for the light source normally to be used with said reflector,- positioning a mask having at least one opening therethrough in front of said reflector to intercept the light rays reflected thereoutline on the screen to thus measure the imperfections in t the shape of the reflector surface.

4. The method set forth in claim 3 and further comprising the step of: measuring the distance from the light source to the center of thereflector when the shadow of the mast: is congruent with the outline on the screen.

5. Apparatus for testing a parabolic reflector comprising, light means approximating a point of light to minimize the divergence of light rays, means within said reflector for moving said light means along the optical axis of said reflector and with respect to said reflector, a mask of opaque narrow strip material of open square configuration in front of said reflector, said mask intercepting a small portion of the reflected beam of said light means, and intersecting at the edges thereof a wide variety of radii of the reflected beam bounded by the shorter distance across the opening of the mask and the greater distance across the outermost corners thereof, and a screen having an exact outline of said mask thereon positioned parallel to and spaced from the opposite side of said mask as said reflector to intercept the same portion of the reflected beam as said mask, said screen receiving the masked beam of said light means whereby the shadow of said mask is brought into congruence with said outline by moving said light means to the focal point of said reflector at which point imperfections of the reflector will appear on the screen as imperfections at the edges of said shadow and as imperfections of the image of the beam in the unmasked portions of the beam.

6. Apparatus for testing a parabolic reflector comprising, light means approximating a point of light to minimize the divergence of light rays, means within said reflector for moving said light means along the optical axis of said reflector and with respect to said reflector, a mask having at least one opening therethrough in front of said reflector, said mask intercepting a portion of the reflected beam of said point of light, a screen having an exact outline of said mask thereon positioned parallel to and spaced from the opposite side of said mask as said reflector to intercept the same portion of the reflected beam as said mask, said screen receiving the masked beam of said light means whereby the shadow of said mask is brought into congruence with said outline by moving said light means to the focal point of said reflector at which point imperfections of the reflector will appear on the screen as imperfections at the edges of said shadow and as imperfections of the image of the beam in the unmasked portions of the beam.

References Cited in the file of this patent UNITED STATES PATENTS 

